1. Field of the Invention
The present invention relates to a method for acylating a hexakis(arylmethyl)hexaazaisowurtzitane. More particularly, the present invention is concerned with a method for acylating a hexakis(arylmethyl)hexaazaisowurtzitane (hereinafter, frequently referred to simply as "WB.sub.6 ") by reductive dearylmethylation in the presence of an acylating agent, which comprises contacting (a) a WB.sub.6 and (b) a heterogeneous reduction catalyst with each other in the presence of (c) an acylating agent and (d) a reducing agent in (e) a solvent for the WB.sub.6 (a), thereby performing a reductive dearylmethylation/acylation reaction of WB.sub.6 (a), wherein there is no contact between WB.sub.6 (a) and heterogeneous reduction catalyst (b) in the absence of any of acylating agent (c) and reducing agent (d). By the method of the present invention, in the production of tetraacylhexaazaisowurtzitane derivatives (which are useful as precursors of a hexanitrohexaazaisowurtzitane utilized for improving the performance of conventional explosives) from a WB.sub.6 by acylation, the decomposition of a hexaazaisowurtzitane skeleton, which is likely to occur at the initial stage of the acylation reaction of a WB.sub.6 as a starting material, can be very effectively suppressed, so that desired tetraacylhexaazaisowurtzitane derivatives can be stably produced in high yield. Therefore, the method of the present invention is commercially advantageous. Further, the method of the present invention is also advantageous in that the lowering of the catalytic activity of the reduction catalyst during the reaction can be effectively suppressed, as compared to the case of conventional methods.
2. Prior Art
As a conventional method for producing a tetraacylbis(arylmethyl)hexaazaisowurtzitane (hereinafter, frequently referred to simply as "WA.sub.4 B.sub.2 "), a method is known in which a hexakis(arylmethyl)hexaazaisowurtzitane (i.e., WB.sub.6) is subjected to reductive dearylmethylation in the presence of an acylating agent, to thereby obtain a WA.sub.4 B.sub.2 (see "Tetrahedron" vol. 51, No. 16, 4711-4722 (1995), International Patent Application Publication Nos. WO96/23792 and WO97/20785 and U.S. Pat. No. 5,693,794).
Further, there is also known a method in which a WA.sub.4 B.sub.2 is subjected to reductive dearylmethylation, to thereby obtain a tetraacylhexaazaisowurtzitane (hereinafter, frequently referred to simply as "WA.sub.4 H.sub.2 ")(see the above-mentioned WO96/23792).
In each of these patent and non-patent documents, the production of a WA.sub.4 B.sub.2 is conducted by a method in which a starting material (i.e., WB.sub.6) and other materials for the reaction (including an acylating agent, a solvent, a catalyst and the like) are charged into a reactor at a relatively low temperature (i.e., from 5 to 25.degree. C.) to obtain a mixture, and then, hydrogen gas as a reducing agent is introduced into the reactor while maintaining the temperature of the obtained mixture at the above-mentioned relatively low temperature, followed by stirring, to thereby perform a reaction (exothermic reaction) wherein the temperature of the reaction system is elevated to a desired level (i.e., from 40 to 70.degree. C.) by heat generated in the reaction. In this method, the WB.sub.6 is subjected to reductive dearylmethylation in the presence of an acylating agent in order to effect the acylation of a WB.sub.6 smoothly while preventing the skeletal decomposition of the WB.sub.6. In this method, since the starting material and other materials for the reaction are charged into the reactor at a relatively low temperature, the occurrence of the thermal decomposition of the WB.sub.6 skeleton can be suppressed. However, even by this method, the skeletal decomposition of the WB.sub.6 cannot be satisfactorily suppressed. In addition, this method is accompanied by disadvantageous side reactions, such as a reductive side reaction in which the acyl group bonded to the hexaazaisowurtzitane skeleton as a result of the acylation of the WB.sub.6 is converted to an alkyl group. Therefore, in this method, the desired product cannot be obtained in a satisfactorily high yield.
In the above-mentioned prior art technique, the reaction is started at a relatively low temperature (i.e., from 5 to 25.degree. C.). However, instead of this technique, it is possible to start the reaction in a manner such that a reaction is started after the temperature of the mixture of the raw material and the reaction reagents is elevated to a predetermined level (i.e., from 40 to 70.degree. C.). As an example of the method for starting the reaction after elevating the temperature of a mixture of the raw material/other materials for the reaction to a predetermined level, there can be mentioned a method in which the temperature of a mixture of a starting material (i.e., WB.sub.6) and other reaction materials (including as an acylating agent, a solvent, a catalyst and the like) is elevated to a predetermined level without addition of hydrogen gas as a reducing agent to the mixture, and then, hydrogen gas is added to the mixture to thereby conduct the reaction. However, this method is disadvantageous in that the skeletal decomposition of the WB.sub.6 markedly occurs, that is, the problems accompanying the above-mentioned prior art technique cannot be solved.
In addition, the above-mentioned prior art technique also has a problem in that the catalytic activity of a heterogeneous reduction catalyst (comprised of, for example, a rare metal of the platinum group) used in the reaction is likely to be lowered. Generally, it is desired that a deactivated catalyst is reactivated so as to recycle the catalyst. However, for reactivating the deactivated catalyst, it is necessary to perform a reactivation treatment which requires a cumbersome operation and a high cost. Therefore, from a commercial point of view, the recycling of the catalyst is not practical. Further, the above-mentioned problem of the lowering of the catalytic activity causes a disadvantage in that it is difficult to perform the reaction by a commercially advantageous continuous process. Therefore, it has been desired to prevent the lowering of the activity of the heterogeneous reduction catalyst during the reaction.
As mentioned above, the conventional method comprising subjecting a WB.sub.6 to reductive dearylmethylation in the presence of an acylating agent has a problem in that it is difficult to stably produce tetraacylhexaazaisowurtzitane derivatives in high yield while surely preventing the decomposition of the hexaazaisowurtzitane skeleton. Further, the conventional method also has a problem in that the lowering of the catalytic activity during the reaction is likely to occur. Therefore, conventionally, it has been difficult to perform the reaction in a commercially advantageous continuous manner.